The concept of the zodiac originated in Babylonian astrology, and was later influenced by Hellenistic culture. According to astrology, celestial phenomena relate to human activity on the principle of "as above, so below", so that the signs are held to represent characteristic modes of expression,[1] or primary energy patterns indicating specific qualities of experience, through which planets manifest their dimension of experience.[2]

The twelve sector division of the ecliptic constitutes astrology's primary frame of reference when considering the positions of celestial bodies, from a geocentric point of view, so that we may find, for instance, the Sun in 23º Aries (23º longitude), the Moon in 7º Scorpio (217º longitude), or Jupiter in 29º Pisces (359º longitude). Beyond the celestial bodies, other astrological points that are dependent on geographical location and time (namely, the Ascendant, the Midheaven, the Vertex and the houses' cusps) are also referenced within this ecliptic coordinate system.

Various approaches to measuring and dividing the sky are currently used by differing systems of astrology, although the tradition of the Zodiac's names and symbols remain consistent. Western astrology measures from Equinox and Solstice points (points relating to longest, equal and shortest days of the tropical year), while Jyotiṣa or Vedic astrology measures along the equatorial plane (sidereal year). Precession results in Western astrology's zodiacal divisions not corresponding in the current era to the constellations that carry similar names,[3] while Jyotiṣa measurements still correspond with the background constellations.[4]

In Western and Asian astrology, the emphasis is on space, and the movement of the Sun, Moon and planets in the sky through each of the zodiac signs. In Chinese astrology, by contrast, the emphasis is on time, with the zodiac operating on cycles of years, months, and hours of the day. A common feature of all three traditions however, is the significance of the Ascendant - the zodiac sign that is rising (due to the rotation of the earth) on the eastern horizon at the moment of a person's birth.

The twelve ecliptic signs. Each dot marks the start of a sign and they are separated by 30°. The intersection of the celestial equator and the ecliptic define the equinoctial points: First Point of Aries () and First Point of Libra (). The great circle containing the celestial poles and the ecliptic poles (P and P'), intersect the ecliptic at 0° Cancer () and 0° Capricorn (). In this illustration, the Sun is schematically positioned at the start of Aquarius ().

While Western astrology is essentially a product of Greco-Roman culture, some of its more basic concepts originated in Babylonia. Isolated references to celestial "signs" in Sumerian sources are insufficient to speak of a Sumerian zodiac.[5] Specifically, the division of the ecliptic in twelve equal sectors is a Babylonian conceptual construction.[6]

By the 4th century BC, Babylonians' astronomy and their system of celestial omens were influencing the Greek culture and, by the late 2nd century BC, Egyptian astrology was also mixing in. This resulted, unlike the Mesopotamian tradition, in a strong focus on the birth chart of the individual and in the creation of horoscopic astrology, employing the use of the Ascendant (the rising degree of the ecliptic, at the time of birth), and of the twelve houses. Association of the astrological signs with Empedocles' four classical elements was another important development in the characterization of the twelve signs.

The body of astrological knowledge by the 2nd century AD is described in Ptolemy's Tetrabiblos, a work that was responsible for astrology's successful spread across Europe and the Middle East, and remained a reference for almost seventeen centuries as later traditions made few substantial changes to its core teachings.

The following table enumerates the twelve divisions of celestial longitude, with the Latin names (still widely used) and the English translation (gloss). The longitude intervals, being a mathematical division, are closed for the first endpoint and open for the second - for instance, 30° of longitude is the first point of Taurus, not part of Aries. Association of calendar dates with astrological signs only makes sense when referring to Sun sign astrology.

A simple diagram displaying the planets' sign positions on May 16th, 2012. The signs are colored according to the associated element. Each planet is represented by a glyph next to its longitude within the sign. Additional symbols may be added to represent apparent retrograde motion(), or apparent stationary moment (shift from retrograde to direct, or vice versa: S).

Empedocles, a fifth-century BC Greek philosopher, identified Fire, Earth, Air, and Water as elements. He explained the nature of the universe as an interaction of two opposing principles called love and strife manipulating the four elements, and stated that these four elements were all equal, of the same age, that each rules its own province, and each possesses its own individual character. Different mixtures of these elements produced the different natures of things. Empedocles said that those who were born with near equal proportions of the four elements are more intelligent and have the most exact perceptions.[7]

Each sign is associated with one of the classical elements,[8] and these can also be grouped according to polarity: Fire and Air signs are considered positive or extrovert, masculine signs; while Water and Earth signs are considered negative or introvert, feminine signs. A modern approach looks at elements as "the energy substance of experience"[2] and the next table tries to summarize their description through keywords.[9][10]

Classification according to element has gained such importance, that some astrologers start their interpretation of a natal chart, by studying the balance of elements shown by the position of planets and angles[12] (especially the Sun, the Moon and the Ascendant).

Each of the four elements manifests in three modalities: Cardinal, Fixed and Mutable.[13] As each modality comprehends four signs, these are also known as Quadruplicities.[8] They are occasionally referred to as crosses because each modality forms a cross when drawn across the zodiac. Christian astrology relates the three qualities to the three aspects of God in the trinity[citation needed].

The combination of element and modality provides a basic sign characterization. For instance, Capricorn is a cardinal earth sign, meaning that it is associated with action (cardinal modality) in the material world (earth element). That can translate into ambition or practical application to the concrete, everyday necessities of life.[17] The next table displays the twelve combinations of elements and modalities.

Psychologically-oriented astrologers often believe that Uranus is the ruler or co-ruler of Aquarius instead of Saturn; Neptune is the ruler or co-ruler of Pisces instead of Jupiter, and that Pluto is the ruler or co-ruler of Scorpio instead of Mars. Some astrologers[20] believe that the planetoid Chiron may be the ruler of Virgo, while other group of modern astrologers claim that Ceres is the ruler of Taurus instead. Other astrologers, still, use the former planets Pallas, Vesta, Juno and Hygiea in their delineations and rulerships, for example Vesta to Taurus and Pallas to Virgo.

Debate continues between those who consider the newly discovered planets as rulers or co-rulers of certain signs and those that do not. Some astrologers do not even use the astrological signs at all (mostly Cosmobiologists and Uranian Astrologers/Hamburg School). Therefore they do not take into account planetary rulerships and the essential dignities when interpreting an astrological chart.

Note that, if one starts from Leo and Cancer, the traditional planetary rulers are arrayed outward in the same order from the sun as they occur in the natural solar system. The Lights ruling Leo and Cancer, Mercury ruling Virgo and Gemini, Venus ruling Libra and Taurus, Mars ruling Scorpio and Aries, Jupiter ruling Sagittarius and Pisces, Saturn ruling Capricorn and Aquarius. The result is a symmetry of traditional rulerships across the 0° Leo/Aquarius axis. Note that modern rulerships, which attribute Pluto as ruler of Scorpio, break this symmetry.

The following table shows both, traditional[19] and modern,[21] rulerships.

A traditional belief of astrology, known as essential dignity, is the idea that the Sun, Moon and planets are more powerful and effective in some signs than others, because the basic nature of both is held to be in harmony. By contrast, they are held to find some signs to be weak or difficult to operate in because their natures are thought to be in conflict. The most important of these categories are Dignity, Detriment, Exaltation and Fall.

Dignity and Detriment : A planet is strengthened or dignified if it falls within the sign that it rules. In other words it is said to exercise Rulership of the sign. For example, the Moon in Cancer is considered "strong" (well-dignified). Seventeenth century astrologer William Lilly compared rulership to a king on his throne, with considerable dignity. If a planet is in the sign opposite that which it rules (or is dignified), it is said to be weakened or in Detriment (for example, the Moon in Capricorn).[22]

In traditional astrology, other levels of Dignity are recognised in addition to Rulership. These are known as Exaltation (see below), Triplicity, Terms or bounds, and Face or Decan, which together are known as describing a planet's Essential dignity, the quality or ability to give of one's true nature. Contemporary traditional astrologers like John Frawley or J Lee Lehman explain further on the concept of Essential Dignity.[22]

Exaltation and Fall : In addition, a planet is also strengthened when it is in its sign of Exaltation. In traditional horary astrology, Exaltation denotes a level of dignity somewhat exaggerated compared to rulership. Exaltation was considered to give the planet (or what it signified in a horary chart) dignity, with the metaphor of an honoured guest – who is the centre of attention but the extent of their ability to act is limited. Examples of planets in their Exaltation are: Saturn (Libra), Sun (Aries), Venus (Pisces), Moon (Taurus), Mercury (Virgo, although some disagree to this classification), Mars (Capricorn), Jupiter (Cancer). A planet in the opposite sign of its Exaltation is said to be in its Fall, and thus weakened, perhaps seemingly more so than Detriment. The Planet in fall is passively rejected or ignored by the sign that it's in. It can be likened to a mayor of a rival city trying to make suggestions to the hosting mayor for how he should run his city: The host mayor finds it difficult to trust him and cannot see how his input could have relevancy to his city. The people of the city feel the same as their ruler. The result is impasse and failure on behalf of both mayors and the city.[22] There is no agreement as to the signs in which the three extra-Saturnian planets may be considered to be exalted.[23]

The following table summarizes the positions described above:

Planet (Symbol)

Dignity

Detriment

Exaltation

Fall

Sun ()

Leo

Aquarius

Aries

Libra

Moon ()

Cancer

Capricorn

Taurus

Scorpio

Mercury ()

Gemini and Virgo

Sagittarius and Pisces

Aquarius

Leo

Venus ()

Libra and Taurus

Aries and Scorpio

Pisces

Virgo

Mars ()

Aries and Scorpio

Libra and Taurus

Capricorn

Cancer

Jupiter ()

Sagittarius and Pisces

Gemini and Virgo

Cancer

Capricorn

Saturn ()

Capricorn and Aquarius

Cancer and Leo

Libra

Aries

In addition to essential dignity, the traditional astrologer considers Accidental dignity of planets. This is placement by house in the chart under examination. Accidental dignity is the planet's "ability to act." So we might have, for example, Moon in Cancer, dignified by rulership, is placed in the 12th house it would have little scope to express its good nature.[24] The 12th is a cadent house as are the 3rd, 6th and 9th and planets in these houses are considered weak or afflicted. On the other hand, Moon in the 1st, 4th, 7th or 10th would be more able to act as these are Angular houses. Planets in Succedent houses of the chart (2nd, 5th, 8th, 11th) are generally considered to be of medium ability to act. Besides Accidental Dignity, there are a range of Accidental Debilities, such as retrogradation, Under the Sun's Beams, Combust, and so forth.

Each sign can be divided into three 10° sectors known as decans or decanates, though these have fallen into disuse. The first decanate is said to be most emphatically of its own nature and is ruled by the sign ruler.[25] The next decanate is sub-ruled by the planet ruling the next sign in the same triplicity. The last decanate is sub-ruled by the next in order in the same triplicity.[26]

While the element and modality of a sign are together sufficient to define it, they can be grouped to indicate their symbolism. The first four signs, Aries, Taurus, Gemini and Cancer, form the group of personal signs. The next four signs, Leo, Virgo, Libra and Scorpio form the group of interpersonal signs. The last four signs of the zodiac, Sagittarius, Capricorn, Aquarius and Pisces, form the group of transpersonal signs.[27]

Dane Rudhyar presented the tropical zodiac primary factors,[28] used in the curriculum of the RASA School of Astrology. The tropical zodiac is the zodiac of seasonal factors as opposed to the sidereal zodiac (constellation factors). The primary seasonal factors are based on the changing ratio of sunlight and darkness across the year. The first factor is whether the chosen time falls in the half of the year when daylight is increasing, or the half of the year when darkness is increasing. The second factor is whether the chosen time falls in the half of the year when there is more daylight than darkness, or the half when there is more darkness than daylight. The third factor is which of the four seasons the chosen time falls in, defined by the first two factors. Thus[29][30]

the 'winter' season is when daylight is increasing and there is more darkness than daylight.[29]

the 'spring' season is when daylight is increasing and there is more daylight than darkness.[29]

the 'summer' season is when darkness is increasing and there is more daylight than darkness.[29]

the 'autumn' season is when darkness is increasing and there is more darkness than daylight.[29]

Unlike the Western or Indian zodiacs, the Chinese zodiac signs are not derived from constellations, and are not assigned to sections of the ecliptic. Instead, Chinese astrological signs operate on cycles of years, lunar months, and two-hour periods of the day (also known as shichen). A particular feature of the Chinese zodiac is its operation in a 60-year cycle in combination with the Five Phases of Chinese astrology (Wood, Fire, Metal, Water, and Earth).[33] Nevertheless some researches say that there is an obvious relationship between the Chinese 12-year cycle and zodiac constellations: each year of the cycle corresponds to a certain disposal of Jupiter. For example, in the year of Snake Jupiter is in the Sign of Gemini, in the year of Horse Jupiter is in the Sign of Cancer and so on. So the Chinese 12-year calendar is a solar-lunar-jovian calendar.

Chart showing the 24 cardinal directions and the symbols of the sign associated with them.

In Chinese astrology the zodiac of twelve animal sign represents twelve different types of personality. The zodiac traditionally begins with the sign of the Rat, and there are many stories about the Origins of the Chinese Zodiac which explain why this is so. When the twelve zodiac signs are part of the 60-year calendar in combination with the four elements, they are traditionally called the twelve earthly branches. The Chinese Zodiac follows the lunisolar Chinese calendar[citation needed] and thus the "changeover" days in a month (when one sign changes to another sign) vary each year. The following are the twelve zodiac signs in order.[34]

Wood: The wood person has high morals, is self-confident, expansive and co-operative, with wide and varied interests and idealistic goals. The direction associated with Wood is East, and the season is spring, which makes it the fixed element for the animal signs Tiger and Rabbit.[35]

Fire: The fire person has leadership qualities, dynamic passion, and is decisive, self-confident, positive and assertive. The direction associated with Fire is South, and the season is summer, which makes it the fixed element for the animal signs Snake and Horse.[35]

Earth: The earth person is serious, logical and methodical, intelligent, objective and good at planning. The direction associated with Earth is Center. The season for Earth is the changeover point of the four seasons. It is the fixed element for the animal signs Ox, Dragon, Goat and Dog.[35]

Metal: The metal person is sincere, has fixed values and opinions, is strong of will, and has eloquence of speech. The direction associated with Metal is West. The season for Metal is Autumn. It is the fixed element for the animal signs Monkey and Rooster.[35]

Water: The water person is persuasive, intuitive, and empathetic. The water person is objective and often sought out for their counsel. The direction associated with water is North. The season for Water is Winter. It is the fixed element for the animal signs Rat and Pig.[35]

The five elements operate together with the twelve animal signs in a 60-year calendar. The four elements appear in the calendar in both their yin and yang forms and are known as the eight heavenly stems. When trying to calculate the relevant year of the cycle in relation to the Western calendar, an easy rule to follow is that years that end in an odd number are Yang (representing masculine, active and light), those that end with an even number are Yin (representing feminine, passive and darkness).[35]